(USA TODAY) - Adam Lanza left only the faintest impression on classmates, neighbors and the people of Newtown before he killed his mother and shot his way into Sandy Hook Elementary, where he killed 20 children and six teachers before turning the gun on himself.

Lanza, 20, skulked through the hallways of Newtown High School in over-sized button-down shirts, eyes perpetually downcast. His name appears a few times on the honor roll published in the weekly Newtown Bee, but his picture is absent from The Newtown Nighthawk yearbook.

"He was very withdrawn," said Tracy Dunn, 20, who graduated from Newtown High School in 2010, Lanza's class.

Dunn, a junior at Massachusetts College of Pharmacy and Health Sciences, said she never saw him talk with anyone or hang out with friends. He spent time with computers and cameras in the technology room and belonged to the Tech Club.

"He would always have his head down walking to class with his briefcase - kind of scurrying," she said. "He never sat down or said anything to kids at his locker. He was just there in the background."

Andrew Lapple, who sat next to Lanza in homeroom their senior year at Newtown High, told the Hartford Courant that Lanza "never really talked at all" and walked the corridors at school clutching his laptop.

"He walked down the halls against the wall, almost like he was afraid of people," Lapple said.

"He was definitely kind of strange, but you'd never think he'd do something like this."

Lanza grew up in Sandy Hook in a sprawling colonial house with his parents, Nancy and Peter, and an older brother, Ryan, 24. His parents divorced in 2009 after a long separation, and his father has remarried. Family friends and relatives say much of his education was home schooling by his mother.

He attended Reed Intermediate School for sixth grade and appears in a 2003 yearbook photo. In the 2005 yearbook for Newtown Middle School, he's listed with the seventh grade without a picture under "camera shy," but he isn't listed in the eighth-grade class the next year. His name surfaces at Newtown High School in 2008 as a sophomore.

Marsha Moskowitz of Sandy Hook drove the school bus that took Lanza to Newtown Middle. She remembers him as "quiet, shy and reserved."

His mother clashed with school officials and eventually removed Adam from public school and home-schooled him, her former sister-in-law, Marsha Lanza of Chicago, told a CBS News affiliate.
Lanza had trouble with her youngest son for years, said her friend Louise Tambascio, owner of My Place Pizza & Restaurant.He was diagnosed with a disorder on the autism spectrum called Asperger's syndrome, she said. Psychologist Elizabeth Laugeson, an assistant clinical professor at the University of California-Los Angeles, has no knowledge of Adam Lanza's case but said, "There really is no clear association between Asperger's and violent behavior."

Nancy Lanza stopped into My Place once or twice a week but rarely talked about her younger son, Tambascio said. Ryan, the older son, bused tables at the restaurant for two years. He is outgoing and personable, she said. The brothers haven't spoken in two years, she said.

"Ryan was the complete opposite of his brother," she said. Adam "always had his face down. He would never look you in the eye."

Police say the three guns used in the massacre were purchased legally and registered to Nancy Lanza, whom friends describe as a gun and shooting aficionado. Tambascio said shooting was "a hobby."

She "had nothing to do with what her son did. She's a good person, goodhearted. She would do anything for you," Tambascio said.

Nancy Lanza "liked the single-mindedness of shooting," her landscaper, Dan Holmes, told The Washington Post. Holmes said she mentioned taking her son to the firing range to practice. Holmes never entered the house or saw her son, but she did once bring an antique rifle outside to show him, he told the Post.

The shooter's father, Peter Lanza of Stamford, said the family is "in a state of disbelief."

"We, too, are asking why," he said in a written statement. "Like so many of you, we are saddened, but struggling to make sense of what has transpired."

The Lanzas' neighbors on Yogananda Street say it's puzzling that on such a close-knit block where residents throw barbecues for newcomers, so few of them knew Adam Lanza or had ever seen him.

"It's a mystery. Nobody knows them, which is odd for this neighborhood," Len Strocchia said. "Everyone knows each other through the children, the school bus. The community
here is kids."

Neighbor Dave Lapp said he had little to tell the FBI and State Police when they called on Friday night. "We walked by their house with the dog every day, and we don't know them. We've never even seen them," Lapp said.

Dunn, Adam Lanza's classmate, fears that may have been at the root of the problem.

"Maybe if someone had tried to reach out to Adam - maybe he needed a friend. Maybe this wouldn't have happened," Dunn said. "He's just one kid who slipped through the cracks."